Keypad backlighting is currently used in many electronic devices such as mobile phones and personal digital assistants. While current keypad backlighting technologies enhance the visible recognition of individual keys, the current keypad backlighting technologies negatively impact the size and functionality of the devices in which they are used.
FIG. 1 depicts a cross-sectional view of a conventional backlighting system 10 using top-emitting light emitting diodes (LEDs) 12. Specifically, the LEDs 12 are wide-angle, top-emitting, component LEDs 12. The LEDs 12 illuminate a stiff light guide panel 14. The LEDs 12 and light guide panel 14 are disposed within a housing having a top plate 16 and a bottom plate 18. Typically, the light guide panel 14 is punched with holes so that keypad plungers 22 can extend through the light guide panel 14. The keypad plungers 22 are coupled to a plunger layer 20 that is a low optical grade material. When a user pushes a button (not shown) on the keypad, the plunger layer 20 and keypad plunger 22 transfer the tactile contact to a switch 24 within the housing. One type of switch 24 is a metal dome of the type known in the art. Unfortunately, this conventional backlighting system 10 has a disadvantage of creating poor brightness uniformity due to noticeable hot spots from the LEDs 12. Also, the thickness of the top-emitting LEDs ranges between 0.35 to 0.60 mm, thereby adding to the thickness of the backlighting system.
FIG. 2 depicts a cross-sectional view of a conventional backlighting system 30 using side-emitting LEDs 32. This backlighting system 30 includes many of the same components as the backlighting system shown in FIG. 1 and described above. In particular, the backlighting system 30 includes a stiff light guide panel 14, a housing having a top plate 16 and a bottom plate 18, a plunger layer 20, a keypad plunger 22, and a switch 24. The light guide panel 14 of this backlighting system 30 is illuminated by side-emitting LEDs 32. Unfortunately, this conventional backlighting system 30 also has a disadvantage of hot spots from the holes 34 in the light guide panel 14 to accommodate the keypad plungers 22. The holes 34 result in localized hot spots and deteriorate the brightness uniformity performance of the light guide panel. The holes 34 also reduce the tactility of the keypad buttons.
A third type of conventional backlighting system (not shown) uses electroluminescent (EL) lighting. EL lighting systems emit light in response to an electrical current passed through an electroluminescent material. However, the rectifier circuitry in EL backlighting systems generates noise which negatively interferes with other electronic circuitry within the backlit device.
In view of this, what is needed is a backlighting solution to overcome the problems of hotspots, thickness, and noise associated with conventional backlighting technologies.